Excited state of peptide nanorings

Peptide nanorings (PNRs) has a closed ring form composed of alternating D- and L-$\alpha $-amino acid residues. Much interest has been attracted owing to their function as a selective ion receptor. Here, based on \textit{ab initio} molecular orbital theory, the excited as well as the ground state structures have been theoretically studied and discussed how the photo-excitation changes the ion-capturing ability with regarding on the two ring skeletons of Extended (E)- and Bound (B)-type PNR geometries. Because the HOMO-LUMO transition is optically allowed, we took into account this single excitation via the unrestricted Hartree-Fock (UHF/6-31G**) calculations. Our results reveal that the HOMO-LUMO single electron transition expands the bore of E-type PNR while narrows that of B-type one. This \textit{ad hoc} treatment, however, causes an overestimation, because the electronic states both near the HOMO and LUMO are \textit{pseudo}-degenerated. An inclusion of other configurations due to the single electron excitation (CIS) relaxes the above geometrical changes predicted by the UHF HOMO-LUMO single excitation. Thus, the ring molecular geometry in the first optical excitation is rather indistinguishable compared with that in the ground state.